What is Maërl?

Maërl refers to a group of free-living coralline red algae belonging to the Class Rhodophyta. The term “Maërl” is a Breton term with there even being a festival about maerl in Brittany. Maërl beds form a complex three-dimensional habitat and have been found to harbour greater diversity of life in comparison to surrounding habitats. As a result they are protected under the EU Habitats Directive, with two maërl forming species, Phymatolithon calcareum (Pallas) W.H.Adey & D.L.McKibbin, 1970., Lithothamnion corallioides (P.L.Crouan & H.M.Crouan) P.L.Crouan & H.M.Crouan, 1867, found in under Annex V. In many cases, maërl is also given protection under Annex 1 of the EU Habitats directive as a sub-feature habitat and as an indicator of a biodiversity hotspot.

Distribution of maërl in the British Isles- NBN Map- Phymatolithon calcareum Common Maërl

UK Marine SACs Document

Maërl Beds in the Fal Estuary with Harbour crab - Liocarcinus depurator

Biogenic Beach Sediments

Biogenic gravel beaches made of dead or fossil maërl deposits can be found adjacent to live maërl habitats.
Trá an Doilín, Ireland

Many different growth forms result in different grain shapes of the thalli.
Phymatolithon calcareum

Intertidal Maërl

Muckinish Bay in County Clare has maerl beds which are only exposed during the low spring tide.

Subtidal Maërl

Subtidally, maërl free living and forms branches


Maërl megaripples

Where currents are strong enough, or due to disturbance due storm-waves, maërl megaripples can form (asymmetric for currents-formed ripples and symmetric wave-formed ripples) Here is some rare footage of maerl mega ripples in Northern Ireland:

Image credits

(1) Maërl beds in the Fal Estuary with harbour crab – Liocarcinus depurator: Image credit to Ross Bullimore
(2) Trá an Doilín, Ireland Seabed Habitats
(3) Phymatolithon calcareum, Image credit to Malcolm Storey
(4) Muckinish Bay, County Clare, Image credit to Seabed Habitats
(5) Maërl Megaripples, Diver footage courtesy of Joe Breen of the Northern Ireland Environment Agency (NIEA)

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